1. Field of the Invention
The present invention relates to an ignition system for an internal combustion engine.
2. Description of the Related Art
Advancement in performance of an internal combustion engine, such as an automobile gasoline engine, has recently been accompanied by use of a so-called multi-ignition engine, in which each cylinder is equipped with a plurality of spark plugs. The multi-ignition engine exhibits excellent ignition performance and is favorably applicable particularly to a lean-burn engine.
When a spark plug is used for a long period of time at a low temperature not higher than 450xc2x0 C.; for example, during predelivery, the spark plug becomes xe2x80x9ccarbon fouledxe2x80x9d (sooted) or xe2x80x9cwet fouledxe2x80x9d (covered with fuel). In such a state, the insulator surface is covered with a conductive contaminant, such as carbon, which causes defective operation. In the above-described conventional multi-ignition engine, the type and polarity of spark plugs mounted thereon have been determined without sufficiently considering the prevention of contamination.
A first object of the present invention is to provide an ignition system for an internal combustion engine having improved ignition performance by attaching a plurality of spark plugs to each cylinder which are less susceptible to contamination. A second object of the present invention is to provide a method for simplifying the electrical configuration of an ignition system having a plurality of spark plugs attached to each cylinder.
The above first object of the present invention has been achieved by providing an ignition system for an internal combustion engine having a multi-ignition cylinder equipped with a plurality of spark plugs serving as ignition sources, characterized in that at least one of the spark plugs is a self-cleaning spark plug capable of removing, by means of discharge spark, contaminants adhering to an insulator surface facing a spark discharge gap of said self-cleaning spark plug.
In an internal combustion engine having a multi-ignition cylinder (hereinafter, also called a multi-ignition-type internal combustion engine), where at least one of a plurality of spark plugs attached to the cylinder is a self-cleaning spark plug as in the case of the present invention, the spark plug becomes unlikely to suffer contamination such as soot accumulation, thereby effectively preventing a problem of engine start-up failure. Even when some spark plugs are contaminated, the self-cleaning spark plug reliably ignites a fuel-air gas mixture. When the temperature of the engine rises sufficiently high, the contaminated spark plugs are cleaned; thus, good ignition can be maintained at all times.
The self-cleaning spark plug can be a surface-gap spark plug comprising a center electrode; an insulator, which is disposed around the center electrode such that an end portion of the center electrode is exposed at an end surface thereof; and a ground electrode. The relative positions of the ground electrode, an end portion of the insulator and the end portion of the center electrode are determined such that a spark discharge gap is defined between the ground electrode and the end portion of the center electrode and such that the discharge gap enables creeping spark discharge across the surface of the end portion of the insulator. The surface-gap spark plug allows a spark discharge to creep across the surface of the insulator, thereby burning an adhering contaminant at all times and thus exhibiting improved resistance to contamination as compared with an air-gap-type spark plug.
Meanwhile, a self-cleaning spark plug, such as a surface-gap spark plug, involves frequent occurrence of a spark which creeps across or attacks the surface of an insulator, and thus tends to suffer so-called channeling, or the surface of the insulator is abraded. Progress of channeling is apt to impair heat resistance or reliability of a spark plug. Channeling is particularly apt to occur during high-speed or heavy-load operation. With the recent trend toward high engine output, there has been demand for spark plugs of excellent durability, and there is a need to prevent or suppress channeling. Channeling can be effectively prevented by employing a high-voltage applicator for applying a discharge-inducing high voltage to the center electrode and the ground electrode of the self-cleaning spark plug such the center electrode assumes a positive polarity. The mechanism disclosed in Japanese Patent Application Laid-Open (kokai) No. 11-135229 illustrates why application of voltage so as to establish the above-mentioned polarity effectively prevents channeling to an insulator.
The above second object of the present invention is achieved by providing an ignition system for an internal combustion engine having a plurality of multi-ignition cylinders, each equipped with a plurality of spark plugs serving as ignition sources, characterized in that:
The multi-ignition cylinders are each equipped with a positive-polarity spark plug, to which a discharge-inducing high voltage is applied such that a center electrode of the positive-polarity spark plug assumes a positive polarity, and a negative-polarity spark plug, to which a discharge-inducing high voltage is applied such that a center electrode of the negative-polarity spark plug assumes negative polarity; and
comprising an ignition coil for generating the discharge-inducing high voltage configured such that a positive end of a secondary coil is connected to the positive-polarity spark plug, whereas a negative end of the same secondary coil is connected to the negative-polarity spark plug.
According to the above configuration, a positive-polarity spark plug and a negative-polarity spark plug share a single secondary coil, thereby reducing the number of ignition coils and thus significantly simplifying the electrical configuration of an ignition system employing multi-ignition cylinders.